Generally, can-type beverage containers are of a two-piece construction, with one piece including an integral body and bottom and the other piece being a separately applied lid. Since such cans are cylindrical, they must be printed or decorated by rolling the required decorative ink onto the can body.
Traditionally, can bodies were decorated in multiple colors using a decorator that sequentially applied colored inks in the desired image to a transfer blanket by way of a separate printing plate for each color. Such a can decorating press is disclosed in U.S. Pat. Nos. 3,223,028 (Brigham) and 3,227,070 (Brigham et al.). The application of the various color inks to the blanket is synchronized by mechanical gears. After the multicolored image has been applied to the blanket, the blanket applies the image to the can in one revolution of the can. The can is mounted on a free spinning mandrel. Although the can may be pre-spun prior to printing, as disclosed in U.S. Pat. No. 4,138,941 (McMillin et al.) during the printing process its rotation is driven by frictional contact with the transfer blanket.
The art work on the aforementioned printing plates is so arranged that each color is separated from the adjacent color by very narrow non-printing areas, known as "trap lines". These "trap lines" serve to confine each color to its own design configuration and prevents undesirable bleeding of one color into another. The inks required for use with transfer blankets must be formulated to have a high tack and a paste like viscosity. The high viscosity ensures that the ink will stay within the trap lines, thus avoiding the bleeding of one color into another. The high tackiness also serves to increase the driving friction between the transfer blanket and the can that is necessary to allow the blanket to rotate the can.
Unfortunately, the such high tack viscous inks are very slow drying and require large curing ovens. Further, the inks emit undesirable solvent vapors into the environment.
Decorating cans using a flexographic process offers several advantages. First, flexographic inks are water based and do not emit significant quantities of volatile organic hydrocarbons. Consequently, they are environmentally benign. Second, they are quick drying and do not require oven curing after application. Third, since they are quick drying, the trap lines between each color image can be dispensed with resulting in a more aesthetically pleasing appearance, as well as the ability to overprint several colors in a "dry trap" process.
Unfortunately, use of flexographic inks presents a number of serious difficulties that have heretofore made them impractical for use in decorating cans in a high speed operation, except in rather limited applications, such as printing random numbers on cans otherwise decorated using the traditional blanket transfer process, as disclosed in U.S. Pat. No. 4,884,504 (Sillars). First, they cannot be applied to a transfer blanket as the inks would run together. Accordingly, in order to utilize the flexographic inks, they must be applied directly to the can using a separate printing plate for each color. Consequently, the point of contact of each printing plate with the can must be precisely in registration with the point of contact of the other printing plates.
Satisfying this precise registration requirement is made more difficult by the fact that flexographic inks are not tacky. The lack of tackiness can cause a friction driven can to slip relative to the printing plate, resulting in an image that is out of register. Consequently, the cans must be positively driven while they are in contact with the printing plate to ensure that the surface speed of the can matches that of the printing plate. The net result is that a decorator utilizing flexographic ink has a number of components which must be precisely indexed and synchronized. Although mechanical gearing can be utilized to properly index and synchronize the components, such gears are subject to wear, causing poor quality decoration.
A second difficulty associated with flexography is that it is difficult to ensure uniform contact pressure of the entire can surface over a single printing plate and difficult to ensure uniform contact pressure between different printing plates. Non-uniform contact pressure results in non-uniform decorating.
It would be desirable to provide a decorator for beverage cans, using a flexographic process, that did not require the use of mechanical gearing to synchronize and index the components and that ensured uniform contact pressure of the printing plates against the cans.